1. Field of the Invention
This invention relates to a liquid phase epitaxial growth technique, and more particularly to an improved sliding scheme liquid phase epitaxial growth method.
2. Description of the Prior Art
In the recent years, semiconductor devices of III-V compound semiconductor materials have been in increasing use as light emitting devices, microwave oscillators and field effect mode transistors The liquid phase epitaxial growth technique is well known as one way of manufacturing such III-V compound semiconductor crystals. This technique involves bringing a solute in a melting solvent under saturation at a predetermined temperature into contact with a semiconductor crystalline substrate and subsequently decreasing the temperature by a fixed value to permit the supersaturated solute to be deposited on the substrate, thereby forming an epitaxial growth layer.
The above sort of liquid phase epitaxial growth method is generally performed by the sliding scheme liquid phase epitaxial growth technique. Various forms of the sliding scheme liquid phase epitaxial growth technique have been proposed: One approach replaces the growth melting solution which was used for the growth of a certain layer with a new growth melting solution by pushing out the former, for the growth of a subsequent layer. It is generally known that semiconductor devices having a plurality of continously grown epitaxial layers on the substrate fabricated through the above method exhibit a good crystalline structure and interfacial condition because the interfaces between the epitaxial growth layers are always protected with the growth melting solution without contacting an atmospheric gas.
With the sliding type liquid phase epitaxial growth technique, mixing of the melting solutions is however unavoidable in pushing out the melting solution M.sub.1 which had been used for the growth with the new melting solution M.sub.2. Assume now for the convenience of explanation that a Al.sub.x Ga.sub.1-x As--GaAs--Al.sub.x Ga.sub.1-x As three-layered structure is to be built up by the expitaxial growth method, and especially, let the growth of the second thin layer of GaAs be considered. It is noted that the melting solution M.sub.1 includes a Al.sub.x Ga.sub.1-x As composition and the melting solution M.sub.2 includes a GaAs composition.
A schematic illustration of the solubility of GaAs at three temperatures (typically, 800.degree. C., 805.degree. C. and 810.degree. C.) when Al is added to a Ga solvent is represented in FIG. 1. If a thin GaAs layer of the Al.sub.x Ga.sub.1-x As--GaAs--Al.sub.x Ga.sub.1-x As structure is to be deposited, it may be considered that the GaAs layer grows at a substantially constant temperature during the course of decreasing the temperature of the furnace because of a very short time of growth of GaAs. Assume that such temperature is 800.degree. C., for example. In FIG. 1, M.sub.1 denotes the point where the liquid phase composition of the melting solution is in equilibrium with Al.sub.x Ga.sub.1-x As at 800.degree. C. and M.sub.2 denotes the point where the same is in equilibrium with GaAs at 800.degree. C. When the melting solution M.sub.2 mixes into the melting solution M.sub.1 in pushing out the latter (i.e., a 1:1 mixture of the melting solutions M.sub.1 and M.sub.2) a melting solution M.sub.3 is identified by the middle point M.sub.3 on the line M.sub.1 - M.sub.2. As is clear from FIG. 1, the melting solution M.sub.3 includes a composition which become saturated at approximately 807.degree. C. in respect of GaAs. Since the temperature of the melting solution is now 800.degree. C., the melting solution M.sub.3 during pushing becomes supersaturated to an extent which corresponds to 7.degree. C., in respect of GaAs.
In the case where the previous melting solution M.sub.1 is pushed out and forcedly replaced by the new melting solution M.sub.2 in the above manner after the growth of a certain layer, the melting solutions vary in composition continously from the point M.sub.1 to the point M.sub.2 along the line M.sub.1 - M.sub.2, while mixing with each other. As the composition varies, the melting solution becomes supersaturated to an extent which corresponds to a region defined by the slant line in FIG. 1 (a region surrounded by the GaAs solubility curve at 800.degree. C. and the line M.sub.1 - M.sub.2), resulting in undesirable sudden deposition of a layer.
Although the foregoing has set forth the displacement of the Al.sub.x Ga.sub.1-x As growth melting solution by the GaAs growth melting solution, an undesirable sudden deposition of a layer is similarly observed when the GaAs growth melting solution is replaced with the Al.sub.x Ga.sub.1-x As growth melting solution.
As noted above, the conventional sliding scheme liquid phase epitaxial growth method entails objectionable sudden deposition of a layer and experiences great difficulties in forming an active layer of less then 0.2 .mu.m necessary for semiconductor lasers or the like under proper control of layer deposition.